THE PROBOSCIS. 47 



the posterior labial plate, the tongue, the anterior labial plate, 

 and the labial appendages or lobes. 



The posterior plate of the labium, ( Plate II, Fig, 6, c, and Plate III, c,) 

 replaces the membrane at the bottom of the groove in the canula. It forms 

 the sides and floor of the mouth. Two lateral rods are interposed between 

 it and the operculum anteriorly ; but behind, it tits against the oper- 

 culum, as far back as that part is incomplete. Where the operculum becomes 

 tubular the labium also becomes closed above, and is continued backward 

 united with the tongue, as a tube which joins the salivary duct. The 

 tongue, ( Plate II, Figs. G and 6a,) is tubular behind and united to the 

 Lujium, but becomes thin and lancet-shaped in its anterior third, before 

 the opening of the salivary tube. Its free extremity is covered with fine 

 setae. 



The posterior plate of the labium is probably part of the mental or 

 second segment. Its inferior surface has a number of muscular fibres in- 

 serted into it, which arise from the mesial line of the mentum. On either 

 side of the posterior plate of the labium is a rod, (Plate II, Fig. 6 b,) united 

 to it in the mature fly, but quite distinct when the insect first 

 emerges from the pupa. These rods support a pair of hooks, (Plate II, Figs. 

 5 and 5a,) which, on comparison with the larval hooks (shown in Figs. 4 

 and 4a in the same plate,) will be found to correspond very nearly to 

 those organs. I am inclined to believe they are the homologues of the 

 mandibles, and therefore the lateral appendages of this segment. The 

 larval hooks are supported by anH-shaped organ, (Fig, 4 c.) which seem 

 to represent the second segment, and I have found the same rods and 

 hooks in all the Diptera which I have examined, in which the larva ai e 

 furnished with hooks. Like the maxillze and labial lobes they are placed 

 on the dorsal aspect, and in front of the segment to which they belong. 

 There is no other representative of the mandibles. 



The third segment of the proboscis (Plate III, A. .,) consists of 

 the margin of the anterior opening of the mouth, which is trian- 

 gular, and of the large fleshy lobes at its extremity. These when at 

 rest are folded against each other, but when they are opened, 

 about two-thirds of their anterior surface forms an oval sucker 

 divided into two parts by the fissure between them. Behind this 

 fissure the remainder of the lobes forms a cavity bounded pos- 

 teriorly by the triangular opening of the mouth. 



The anterior surface of the lobes is channelled by a series of 

 canals, kept open by incomplete rings, called false tracheae, 

 which open internally into the cavity between the lobes and so 

 into the mouth ; these form a fine strainer through which the in- 



